Glaciers are formed when snow remains
in an area year round and eventually accumulates to the point
of where it turns to ice. Every year, new snow layers bury and
compress the previous snow layers. This forces the snow to re-crystallize,
forming grains similar to sugar grains. The grains then grow
larger and larger and the air pockets between the grains get
smaller, meaning that the snow is becoming more tightly packed.
After approximately two winters, the snow turns into a
special state of water called firn, intermediate between snow
and ice. The density of firn is roughly half of liquid water. Eventually, the larger ice crystals have become
so compressed that the air pockets in between them have become
very small. This is called glacial ice. In old glacier ice, the
crystals can be several inches long. This process takes over
a hundred years in most glaciers. Glacial ice, because
of its density and ice crystals, often takes a bluish or even
green hue. Glaciers can also be formed
from ice sheets. Ice sheets are large sheets of ice that originally
form over land and extend out into a body of water. Because the
ice over the body of water is not as dense, it stays above it.
As time passes, a piece of the ice sheet can break off and become
a glacier.

Glaciers are formed from:

Ice Sheets-These are found only in Antarctica
and Greenland and are huge masses of glacial ice and snow that
cover over 50,000 sq. kilometers. The ice sheet on Antarctica
is over 4200 meters thick in some places and covers nearly all
the land features except the Transantarctic Mountains that reach
above the ice.

Ice Shelves-These occur when ice sheets
extend over the sea, floating on water. They range in thickness
from a few hundred meters to over 1000 meters. These surround
all of Antarctica with the largest one being the Ross Ice
Shelf. Named after Sir James Clark Ross who discovered it in
1841, it covers over 500,000 square kilometers

Ice Caps-These are mini ice sheets that
cover less then 50,000 square kilometers. They form mostly in
polar and sub-polar regions and occupy high and relatively flat
regions.